add systemd-journal.plugin to gitignore (#15743)

.gitignore

@@ -80,6 +80,9 @@ collectors/ebpf.plugin/includes/
 debugfs.plugin
 !debugfs.plugin/
 
+systemd-journal.plugin
+!systemd-journal.plugin/
+
 # protoc generated files
 *.pb.cc
 *.pb.h